1. Field of the invention
This invention pertains to the tracking, collection, and concentration of direct sunlight by optical and mechanical means, for the purpose of solar energy utilization. The invention may be employed in conjunction with any one of several known means of solar energy utilization, such as thermally-powered production of mechanical or electrical energy, direct photovoltaic generation of electrical energy, or illumination of core building spaces using beamed or piped daylight.
2. Description of prior art
Solar collector types are broadly classified as nontracking (stationary) or tracking (movable). Tracking collectors are classified as "one-axis" or "two-axis" trackers, according to the number of mechanical degrees of freedom employed by the tracking mechanism. [The tracking motion is usually rotational, but we will also apply this classification when non-rotational (e.g., translational) motion is employed.] Tracking collectors usually function to collect direct sunlight entering a large entrance aperture and concentrate it into an exit aperture (i.e., a receiver aperture) of significantly smaller area dimension. The maximum attainable area concentration (ratio of entrance aperture area to exit aperture area) is limited by the number of mechanical degrees of freedom employed by the collector. Non-tracking collectors cannot practically attain concentration levels greater than few suns. A one-axis tracking collector can produce a line focus with much higher area concentration: For north-south (i.e., declination) tracking the theoretical maximum is about 170X; for east-west (i.e., ascension) tracking the limit is about 420X. A twoaxis system can produce a point focus with a theoretical area concentration limit of about 45,000X. [Greater concentration levels can be achieved if the receiver is immersed in a medium having a high refractive index. In addition, high concentration can in principle be attained without active tracking by using fluorescent optical materials which alter the color of the transmitted radiation, but fluorescent systems are not currently practical.]
Two-axis tracking collectors are usually pedestal-mounted on altitude-azimuth tracking bearings. Examples of such systems include heliostats and paraboloidal dish collectors. Due to the single-pedestal mounting's inherent instability and sensitivity to torque loading, it is difficult with these systems to achieve accurate tracking under normal wind loading conditions and to assure survivability under storm conditions without making the system's mechanical elements exceedingly massive, bulky, and expensive. One-axis tracking systems such as the parabolic trough and the linear Fresnel lens have improved mechanical stability, but at the expense of greatly reduced flux concentration.
A number of novel tracking mechanisms have been developed to attempt to overcome the mechanical difficulties encountered with conventional tracking systems. (See Ref. 1 and Chapter 7 in Ref. 2 for a review of novel tracking mechanisms.) The "SLATS" system uses a segmented array of parallel strip mirrors, each rotatable about its own independent axis, in place of a single monolithic mirror unit. Another device, the Fresnel-Belt Solar Collector, achieves one-axis tracking by scrolling a flexible Fresnel mirror across a stationary collector aperture (Ref. 3). The Multielement Optical Panel (Ref. 4) achieves one-axis tracking by making slight adjustments in the relative alignment of two parallel panels bearing linear lens arrays. Two other devices, the Fixed Mirror Solar Concentrator and Spherical Bowl Collector, achieve tracking by moving the receiver in relation to a stationary mirror.
The novel systems mentioned above have various merits in relation to conventional tracking systems, but they also have significant limitations (e.g., optical losses from shading and blocking, optical aberrations, stringent tolerances on surface geometry and mechanical positioning). A primary drawback of these systems is that they are all line-focus devices, and cannot compete with point-focus concentrators in terms of their achievable flux concentration.